fuel mileage seems to be going down....
 

i have a 06 quad cab 4x4 6 speed since new. no mods, ever since changing to the dodge alum. wheels from stock chrome steel wheels and stock tires to stock size bfg all terrains, my fuel mileage has gone down. all highway b4 wheels and tires was 600 miles per tank now luck to get 500- 550 may not be a big deal but....... oil test done, came back great, changed air filter, fuel filter, checked air pressure, driven 5000 plus miles and seems to be only getting worse, lie o meter says 21.6 when long streches on hyw. always drive at 2000 rpms or less at speed. truck has 101000 on it. any ideas? thanks

The tires could affect mileage with a slightly more open tread pattern but not that much.
You could be also getting winter blend fuel. That will have an affect on mileage too.
Neither one should cause that much of a drop though.

Mine went down .5 to .75 mpg hand calculated when I changed my tires. Same brand/size tires. I attributed it to the new ones having more tread, thus a slightly larger diameter.

.5/19mpg = 2.5% difference in mpg.
Circumference = 2pi times the radius.
If radius changed by 1.25% then my initial thought would be right
Measured my tires.
Radius = 17"
Tread depth change = .3"
.3/17 = 1.7%
In your case, mpg change was 8%. Maybe you changed brands of tires and the radius difference was .68inches but that seems like a stretch. Maybe that combined with the tread pattern combined with fuel blend??

Just thinking out loud, explains my slight drop.

The usual places to start (based on records, not assumptions about mpg) are:

Tire pressure (according to load; inside DODGE range); change in tire spec from stock?
Alignment (plus wheel balance)
Steering wander
CAC
Filters & fluids

Change in truck use is another. Longer idle times, for example. What is your average (calculated) mph? Has it changed?

.

Could be you got some bad fuel... Or the station is getting from a different source...

thanks for suggestions, ive tried diff. fuel, air pressure, clutch doesnt slip just wondering if there is too much "slop" still have origial clutch u joints, ect... omly thing not factory is tires and shocks